This invention relates generally to improvements in providing auxiliary electrical and hydraulic power sources for aircraft systems. One improvement concerns a compound hydraulic motor/generator mechanically coupled to an electric motor/generator. The device is termed a Compound electro/hydraulic Motor Generator Pump, with the acronym CMGP. This system allows either electric power to be supplied from a hydraulic power source, or hydraulic power from an electric power source. A second improvement concerns the combination of the CMGP with a ducted ram air turbine drive, defining means to provide continuous electrical and hydraulic power to the aircraft in the event of total failure of all engine driven power sources.
Modern aircraft increasingly rely on powered systems to provide the forces for aircraft control. Early powered systems relied on various forms of mechanical backups, such as traditional cables, to allow pilots to retain control of the aircraft when the powered systems failed. These mechanical backups have many drawbacks in terms of aircraft complexity, weight, space allocation and energy efficiency. Mechanical backup systems become especially cumbersome when integration with modern computer controlled fly-by-wire systems is desired. For this reason many modern aircraft are dispensing with mechanical backup in favor of purely powered controls. These aircraft may have electronically controlled, hydraulic powered surface actuators, electronically controlled electric powered actuators, or combinations of both.
With powered controls, it is required that sufficient power always be available to maintain control of the aircraft in the event of foreseeable failures. The certifying authorities use the term “extremely improbable” to describe that no failure, or series of failures, can result in the loss of the aircraft. For purposes of design, “extremely improbable” is less than 1 occurrence in 1 billion flight hours (10−9). Power to control the aircraft must be shown to be continuously available in the event that all engines, including auxiliary power units (APU), are failed. This requirement has manifested itself in incidents where aircraft have exhausted their fuel supplies, or atmospheric effects such as volcanic ash have resulted in all engine/APU's flaming out. In some aircraft where dispatch reliability is a paramount requirement, the reliability hurdle is further increased by requiring that it be acceptable to dispatch the aircraft with one power system not functioning, typically a failed engine driven generator. This requirement means that with one electric generator (or hydraulic pump) failed, the aircraft must meet the stringent 10−9 requirement.
Prior to these requirements, transport category aircraft typically were fitted with one hydraulic pump and one electrical generator on each engine. A flight operable APU could supply a third electrical generation source, and backup hydraulic power was supplied by one or more electric motor driven hydraulic pumps (ACMP). For two engine aircraft operating under ETOPS (extended over water operations), the need to assure continuous electrical power (both for electronics and the ACMP's) resulted in using flight operable APU's, which must be running continuously during extended over-water flight legs. APU's are relatively in-efficient at generating electrical power, thus this system results in reduced range and fuel economy, and increased exhaust emissions.
Regardless of how many engine driven generators, however, the requirement to supply un-interrupted power in case of all engine and APU failure results in adding a Ram Air Turbine (RAT). The RAT is typically a windmill type propeller deployed outside the aircraft which can drive either an electric generator, hydraulic pump, or both. Because the system must generate sufficient power at relatively low airspeeds, the drag of the RAT and loads become excessive unless it is stowed within the aircraft during all normal flight. This renders the RAT inoperable except for emergency, which incurs two drawbacks. First, the weight, cost, and complexity of the RAT must be borne by the aircraft even though it is virtually never used. Second, the contribution the RAT makes to overall reliability in a statistical Hazards and Safety analysis is limited due to it not normally being operated. There is always an uncertainty about its “availability” since it typically cannot be given a preflight or an in flight check. To improve its statistical availability in the hazard analysis, some operators are requested to occasionally deploy the RAT at the end of a mission to establish that it works. Unless outfitted with a motorized retraction system, the RAT must be re-stowed by the ground crew prior to the next flight.
Another drawback of the conventional external windmill type RAT is that it is not ideal for supersonic flight, and although possible to design for deployment supersonically, the technical challenges of the weight from high drag loads and preventing windmill overspeed makes it less practical for supersonic aircraft.